In gaseous flows, the phenomena of compression exists and has a large effect. It allows the number of molecules for a given volume to change with pressure and temperature as well as with composition. Therefore, it is desirable to make natural gas sales transactions either by mass, energy, or at standard pressure and temperature conditions. In the U.S., for example, the standard pressure and temperature of gas is stated as 14.7 psia and 60.degree. F. for many transactions. Delivery calculations state the flow adjusted to correspond to these base conditions even though the actual gas in the transaction is probably at a different pressure or temperature. A piece of equipment designed to accomplish the task of converting a measured volumetric flowrate to a base volumetric flowrate at a defined pressure and temperature is referred to as a "volume corrector".
In the traditional method of gas measurement, a volume correction ratio ##EQU1## is determined from the pipeline gas flow temperature, pressure, and composition using the following relation: ##EQU2## where Q.sub.f is the measured volumetric flowrate of the pipeline gas through the pipeline, T.sub.b and P.sub.b are the base condition temperature and pressure (e.g. 14.7 psia and 60.degree. F.), T.sub.f and P.sub.f are the measured flow temperature and pressure of the pipeline gas in the pipeline, Z.sub.b and Z.sub.f are the supercompressibility factors at the base condition and the flow condition, respectively, and Q.sub.b is the base condition volumetric flowrate. Such a calculation is typically carried out in a flow computer.
Using the relation in Eq. (1) to compute base condition volumetric flowrate Q.sub.b requires high accuracy in the measurement of the flow temperature T.sub.f and pressure P.sub.f. This requires that pressure and temperature sensors for monitoring P.sub.f and T.sub.f be calibrated frequently.
The ratio ##EQU3## in Eq. (1) presents even more difficulties. The composition of the gas is normally measured by gas chromatography and the supercompressibilities, Z.sub.b and Z.sub.f, are estimated from either the virial equations of state, or from pre-calculated correlations such as NX-19 or the more recent Gergg Equations. Alternatively, a meter that measures heating value, relative density, %CO.sub.2 and %N.sub.2 can be used to calculate the ratio ##EQU4## This is because the Gergg Equations in their short form allow calculation of the ratio ##EQU5## from these parameters.
Knowledge of the values of the virial coefficients of particular gas compositions is quite limited so calculation of supercompressibility from the virial equation of state is not always possible. The Gergg Equations and NX-19 correlation are mathematical models obtained by mapping known and measured properties. The Gergg Equations, in particular, are very good over a wide range of compositions. Use of the Gergg Equations, however, requires either a chromatograph or a special meter to measure the properties needed to solve the short form Gergg Equations, both of which are expensive.
It is, therefore, difficult to obtain accurate measurement of the supercompressibility ratio ##EQU6## in a cost effective manner.
Each of the measurements discussed above (volumetric flow, temperature, pressure, composition, and/or energy content) also introduce an opportunity for measurement error. While the Gergg Equations are regarded as accurate, the aggregation of measurement errors can be quite substantial and can distort calculations. To minimize measurement errors, each piece of instrumentation must be maintained and calibrated periodically. But, even then, additional inaccuracy can accrue in the flow computer from calculations or inaccurate formulas or correlations.
The result is that present day equipment cannot accurately measure energy flowrates, or volume correction ratios ##EQU7## in a cost effective manner.
The present invention alleviates the need to compute the supercompressibility of the pipeline gas. It also alleviates the need to measure the absolute temperature, absolute pressure, and composition of the pipeline gas. The present invention, therefore, operates much more accurately to determine energy flowrates, volume correction ratios ##EQU8## and adjusted or base condition volumetric flowrates Q.sub.b.